There are a few different things you will need to do with images for your online shop before you upload them to get a good presentation of your products. All the following tasks to do with images will be explained using image manipulation software called The Gimp. GIMP stands for GNU Image Manipulation Software.
Although it does hundreds of different things, you will only need to learn about three to get your pictures looking right. There are a few different reasons why I use The Gimp in all the tasks covered: It's very good, it's free and it works on many different operating systems (PC, Mac, Unix). You should download it first from the links below before continuing with the rest of these questions.
Installing on Windows: Download from http://gimp-win.sourceforge.net/stable.html. Basically download it and double click on the icon to install.
Installing on Mac: Bit more involved, download from http://gimp.org/macintosh. Recommended that you watch this YouTube video tutorial on installation: http://www.youtube.com/watch?v=JePWp3qhi6M